1. Field of the Invention
This invention relates to improvements in energy absorption type steering apparatus. As used herein the term "energy absorption type steering apparatus" refers to a steering apparatus in which a steering shaft with a steering wheel mounted on its upper (rear) end and having its lower (forward) end coupled via a joint to a steering gear is rotatably supported on a hollow steering column mounted on vehicle such as to permit only forward and downward movement, and an energy absorption unit consisting of a curved plate or the like is provided between the steering column and vehicle, such that with downward forward movement of the steering shaft and steering column during a collision, the plate member of the energy absorption unit undergoes plastic deformation to absorb the energy of collision.
2. Related Background Art
An energy absorption type steering apparatus of this type is disclosed in Japanese Utility Model Laid-Open No. 53-156230. In this apparatus, an energy absorption plate having a folded portion has one end secured to a portion of vehicle and the other end secured to a steering column. When the steering column is displaced forwardly with an impact force at the time of a collision of the vehicle, the folded portion of the energy absorption plate undergoes plastic deformation to absorb the energy of impact.
As an other example, according to Japanese Patent Application Laid-Open No. 63-46972, a W-shaped deformable wire is locked to a steering column and accommodated in an energy absorption box secured to a vehicle portion such as to be restrained by upper and lower walls and left and right side walls, and at the time of a collision the folded end of the deformable member is brought into contact with an end wall of the energy absorption box to cause plastic deformation of the wire.
In the prior art structure disclosed in Japanese Utility Model Laid-Open No. 53-156230 there is provided no means for restricting the state of plastic deformation of the folded portion. Therefore, it is difficult to provide a fixed plastic deformation resistance of the energy absorption plate for a displacement stroke of the steering column.
In the case of the structure where the deformable wire is accommodated in the energy absorption box, the plastic deformation of the deformable wire can be regulated or controlled by the energy absorption box. Thus, the energy of collision can be reliably absorbed at a fixed rate. In addition, the deformable wire can be readily obtained with a process of bending a single wire. Therefore, the structure is low in cost and is advantageous in that it can be disposed in a narrow space. However, the load characteristic (displacement of column versus deforming force of the deformable wire) is substantially constant at the time of energy absorption. In order to let the deformation be proportional to the displacement of the steering column, the deformable wire should be produced from a non-uniform diameter wire having progressively increasing diameter. This poses difficulties in manufacture.